The invention relates generally to a method and apparatus for producing photoreceptors and, more particularly, to a method and apparatus for producing the hollow drum selenium photoreceptors used in reproduction machines.
The photoreceptors used in reproduction equipment generally are composed of three components; (1) the conductive substrate, usually aluminum, (2) an insulating layer of aluminum oxide, usually several hundred angstroms thick, and (3) the photosensitive thin film, usually selenium or selenium alloy approximately fifty microns thick. The primary manufacturing process steps for producing the photoreceptors include: etching and cleaning the substrate, growing the aluminum oxide layer, usually in an oven, cleaning again, and finally the deposition of the photosensitive film. The etching and cleaning process is typically quite involved and requires several cleaning and rinsing tasks, including deionized water final rinses. The thermal conditioning of the photoreceptor substrate during the process is important as it defines the solidification and crystalization characteristics of the selenium or selenium alloy photosensitive material being deposited on the substrate. These characteristics in turn determine the photosensitivity of the receptor as well as such optical properties as spectral (color) response, and color peaking.
The above noted selenium and selenium alloy coating process conventionally employed to produce photoreceptors is quite well known and the design and manufacture of apparatus to perform the process, although complex, is generally within the state-of-the-art. However, one nagging problem has been an inability to consistently heat the aluminum substrates to the proper temperature and uniformity prior to the deposition of the selenium coating. The failure to achieve coating temperature objectives, can render production equipment virtually useless.
Many coating systems in the past were designed with fluid systems to heat a drum supporting mandrel and relied on conduction heat transfer to heat drum substrates. Such systems function reasonably well in air as the variations in the contacting surfaces are filled by a gaseous interface that assists the transfer of heat. In a vacuum, however, the actual contact area (and thus the heat transfer area) is a series of points, the total area of which is so small that the heating time is very long, and when coupled with any losses, markedly limits or prohibits uniform heating.
In attempts to improve the heating portion of the process, early workers in the field investigated the then evolving gaseous plasma technology as an energy source. Producing discharge to a substrate functioning as a cathode produced very uniform heating, but the plasma energy was too low to produce reasonable heating rates. After investigating the potential of confining the plasma to increase its energy in the immediate area of the substrate, the concept was abandoned due to the difficulty and potential cost of developing a machine of this nature.
The result of this early plasma research suggested that a viable solution existed if the energy could be increased. This fact moved the research toward an older and more thoroughly understood technology, electron bombardment. By controlling the voltage and current of a tungsten filament in a vacuum, it is possible to control the emission of electrons. It is further possible to direct the path of these electrons by means of proper grounding, magnetic fields and/or properly charged shields. Electrons can be emitted in sufficient quantity to achieve heating rates considerably greater than the ion bombardment of normal plasmas. By properly designing the emitting filament relative to the substrate geometry and rotating the substrate past the filament it was found that the required uniformity could be achieved. Unfortunately, the laboratory experiments did not scale up to a production machine very easily. Great difficulty was encountered in controlling the tremendous power supplies required to power the filaments. The systems were prone to generating high voltage arcs as they become more contaminated by each production cycle. The arcs very typically would backtrack through the system wiring burning out electrical components and controls. The production time of electron bombardment equipment is often less than the system down time.
The tungsten filaments required for electron bombardment systems also are a source of considerable problems. The filaments are typically 1/32" to 3/32" in diameter and from 30" to 90" long. They tend to become brittle after only a few heating cycles. Failure usually results in the destruction of some substrates and unacceptable coatings of all the substrates associated with that filament. The thermal expansion mechanism supporting the filament tends to further complicate the filament failure.
The object of this invention, therefore, is to provide an improved system for producing the selenium coated photoreceptors used in reproduction equipment.